Fluid flow measurement device

ABSTRACT

The invention is directed to a device for obtaining flow rate measurements including a sensor assembly and a housing. The sensor assembly includes a body defining a first fluid flow passage having an inlet, an outlet, a flow restricting element in the first fluid flow passage between the inlet and the outlet, an upstream fluid pressure sensor, a downstream fluid pressure sensor, an upstream signal contact connected to the upstream fluid pressure sensor, and a downstream signal contact connected to the downstream fluid pressure sensor. The housing has an upstream portion defining an upstream port, a downstream portion defining a downstream port, and a probe access port configured to provide access of a probe to at least one of the upstream signal contact and downstream signal contact. The housing can also define a second fluid flow passage in parallel with the first fluid flow passage. The device can be disposable.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 10/978,866 filed onOct. 29, 2004, now allowed, which is a continuation of Ser. No.10/442,575 filed on May 21, 2003, now U.S. Pat. No. 6,813,964

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flow sensor device to obtain flowcharacteristics of a fluid flow system, such as a system used inadministering a beneficial agent to a patient. Particularly, the presentinvention is directed to a flow measurement device including first andsecond pressure sensors in a flow passage to measure a flow ofbeneficial agent and, optionally, the presence of air in the fluid flowsystem. The invention also includes a related system and method forobtaining such flow characteristics.

2. Description of Related Art

When administering a predetermined amount of a beneficial agent to apatient over an extended period of time in liquid form, it isbeneficial, if not necessary, to obtain and monitor relevant flowcharacteristics such as flow rates and the presence of air. Whilemethods for obtaining such information have existed for a long time, todate, no reliable low cost systems have been developed for disposableuse.

For example, fluid flow measurements within a disposable IV fluid lineor similar feed set generally have not been financially and technicallyviable up to this point in time. Low cost electronic flow sensors haveexisted for some time, but have to date not presented a viablealternative for solving this problem. Limitations to commercializationof such a device have included inadequate dynamic range of low-cost flowsensor systems and the unacceptable costs of total sensor assembly.

One problem with making flow sensors low cost is in the manufacturingprocess. Silicon chips typically are wire-bonded to a lead frame that isencapsulated and soldered to a printed circuit board. This configurationrequires the manual step of welding wires from the chip to the leadframe, which can result in significant additional manufacturing costs.

Likewise, there has been a long-felt need in the medical field for aneconomical and reliable system to detect the presence of air in IV linesor other medical feed sets. Typically, the presence of air in a fluidline has been sensed externally to the fluid path using a separateultrasound or optical sensor that must communicate through thedisposable tubing or molded component of the fluid path. The ultrasoundapproach may be subject to misalignment and other geometry changes thatcan impact the signal conduction around and through the fluid inside thetubing or other components of the disposable fluid path. The opticalapproach requires specific molded geometries within the fluid path thatare reflective or conductive depending on the presence of air or liquid.These systems are subject to variability in and interfacing to thedisposable fluid path. Also, the added cost of this air detection systemis an impediment to its widespread adoption.

Thus, there remains a need in the art for a reliable fluid flowdetection system that is sufficiently inexpensive to allow use indisposable applications. There is also a continued need for aninexpensive and reliable system to detect the presence of air in fluidsystems, such as IV lines and feed sets.

SUMMARY OF THE INVENTION

The purpose and advantages of the present invention will be set forth inand apparent from the description that follows, as well as will belearned by practice of the invention. Additional advantages of theinvention will be realized and attained by the methods and systemsparticularly pointed out in the written description and claims hereof,as well as from the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described, the invention isdirected to a device for obtaining flow characteristics of a fluid flowsystem.

The device includes a sensor assembly. The sensor assembly includes abody defining a first fluid flow passage having an inlet and an outlet,and a flow restricting element located along the first fluid flowpassage between the inlet and the outlet. An upstream fluid pressuresensor is provided to sense an upstream fluid pressure at an upstreamlocation in the first fluid flow passage between the inlet and the flowrestricting element. The sensor assembly also includes a downstreamfluid pressure sensor to sense a downstream fluid pressure at adownstream location in the first fluid flow passage between the flowrestricting element and the outlet. The sensor assembly also includes anupstream signal contact connected to the upstream fluid pressure sensor,and a downstream signal contact connected to the downstream fluidpressure sensor.

The device also includes a housing. The housing has an upstream portionand a downstream portion. The upstream portion of the housing defines anupstream port in fluid communication with the inlet of the sensorassembly. The downstream portion of the housing defines a downstreamport in fluid communication with the outlet of the sensor assembly. Thehousing also defines a probe access port configured to provide access ofa probe to at least one of the upstream signal contact and downstreamsignal contact.

In accordance with another aspect of the invention, the housing has atleast one registration surface configured to ensure proper registrationof the device with a fluid flow system. The registration surface ensuresthe upstream port is aligned with a fluid source. The registrationsurface can include a surface configuration on the upstream portion ofthe housing that is different from a surface configuration on thedownstream portion of the housing. In accordance with one aspect of theinvention, the registration surface includes at least one planarsurface. The registration surface can also include a detent.

In accordance with a further aspect of the invention, the housingdefines a cavity of predetermined shape, and the sensor assembly has acorresponding shape so as to be received by the cavity. The cavity hasat least one surface, and the surface can include at least one recess toreceive a material to hold the sensor assembly within the cavity. A capcan further be positioned in the cavity proximate to the sensorassembly. The housing can have a connector, such as a Luer connector ora flange, proximate to at least one of the upstream port and thedownstream port for connection with the fluid flow system.

In accordance with another aspect of the invention, the housing candefine a second fluid flow passage therethrough. The second fluid flowpassage can be arranged for fluid communication in parallel with thefirst fluid flow passage between the upstream port and the downstreamport. A valve can further be provided for selective flow through thesecond fluid flow passage. For example, the valve can be formed as acompressible wall member defining at least a portion of the second fluidflow passage. The compressible wall member can be formed from anelastomeric material. In a preferred embodiment, the second fluid flowpassage has a first transverse dimension and a second transversedimension perpendicular to the first transverse dimension. Preferably,the first dimension is smaller than the second dimension so as to bemore readily compressible. Preferably, the cross section of the secondfluid flow passage has an ellipsoidal shape with a small radius at eachapex of the ellipse to facilitate compression of the second fluid flowpassage.

In accordance with another aspect of the invention, a fluid sensorsystem is provided. The system includes a device for obtaining flow ratemeasurements as described above, as well as a probe to receive signalsrepresentative of a fluid flow characteristic and a processor to processsuch signals. The probe can include a connector body having apredetermined shape, such as a wedge configuration, wherein the probeaccess port has a corresponding shape to ensure proper alignment of theprobe with at least one of the upstream signal contact and downstreamsignal contact. The probe also includes a plurality of leads. At leastone lead is provided for communication with the upstream signal contactand at least one lead is provided for communication with the downstreamsignal contact. At least one lead on the probe is configured to wipeacross at least one of the upstream signal contact and the downstreamsignal contact. Preferably, the housing is configured to provide contacton one longitudinal surface and one vertical surface of the housing andprovide for adequate force to ensure contact between the lead on theprobe and the upstream signal contact and the downstream signal contact.The signal contacts can be in close proximity to registration surfaceson the outside of the housing that are engaged with an external clampassembly that is also referenced to the probe.

In accordance with a further aspect of the invention, the system furtherincludes a fluid flow line in communication with a fluid source. Alocking mechanism preferably is provided to mate the housing with thefluid flow line. The locking mechanism has an unlocked condition forreceipt of the housing, a first locked condition to align the housingwith the fluid flow line and a second locked condition to position theprobe in the housing. Additionally, if a second fluid flow passage witha valve is defined in the housing as described above, the system canfurther include an actuator to change the valve from the first conditionto the second condition when the locking mechanism is moved from thefirst locked condition to the second locked condition. The actuator caninclude a protrusion to compress the elastic wall member. In oneembodiment of the invention, the protrusion is a pin.

In further accordance with the invention, the fluid source includes apump connected to the fluid flow system to selectively pump fluidthrough the first fluid flow passage. The processor is configured tocontrol the pump in response to signals obtained by the probe from thesensors.

In further accordance with the invention a method of obtaining flowmeasurements is provided. The method includes providing a device forobtaining flow rate measurements as described above; directing a fluidflow through the first fluid flow passage; obtaining a signalcorresponding to the fluid pressure in the first fluid flow passage atthe locations of the upstream fluid pressure sensor and the downstreamfluid pressure sensor; and determining a flow characteristic based uponthe signal.

In accordance with a further aspect of the invention, the determiningstep includes determining the pressure difference between the upstreamand downstream fluid pressure sensors. The determining step can furtherinclude calculating a flow rate of fluid through the first fluid flowpassage based on the pressure difference.

In accordance with another aspect of the invention, the determining stepincludes detecting the presence of air in the first fluid flow passage.The step of detecting air in the first fluid flow passage can includeidentifying convergence and specific waveforms of the signal receivedfrom the upstream fluid pressure sensor and the signal received from thedownstream fluid pressure sensor.

In accordance with yet another aspect of the invention, a method isprovided further including the steps of intermittently pulsing the fluidthrough the first fluid flow passage and determining the amount of fluiddelivered with each pulse by detecting the fluid pressure in the firstfluid flow passage using the upstream fluid pressure sensor and thedownstream fluid pressure sensor.

In accordance with still another aspect of the invention, a method isprovided wherein the housing provided by the housing step includes asecond fluid flow passage and a valve for selection of flow through thesecond fluid flow passage. The valve has a first condition to allow flowthrough the second flow passage and a second condition to prevent flowthrough the second flow passage. The method further includes the step ofopening the valve to increase flow through the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(c) are a side view and cross-sectional views, respectively,of a first representative embodiment of the device for obtaining flowcharacteristics in accordance with the present invention.

FIGS. 2(a)-2(c) are a plan view, a cross-sectional side view, and an endview, respectively, of the device of FIGS. 1(a)-1(c).

FIG. 3 is a perspective view of the device of FIGS. 1(a)-1(c).

FIGS. 4(a)-4(d) are a plan view, side view, and cross sectional endviews, respectively, of a cap portion for use with the device of FIGS.1(a)-1(c).

FIGS. 5(a)-5(b) are cross sectional end views and an enlarged detail ofthe device of FIGS. 1(a)-1(c).

FIG. 6 is an enlarged view of a selected portion of the device forobtaining flow rate measurements of FIGS. 1(a)-1(c).

FIGS. 7(a)-7(e) are a side view, a plan view and section views of asecond representative embodiment of a flow measurement device inaccordance with the present invention after a first stage of amanufacturing process.

FIGS. 8(a)-8(d) are a plan view and section views of the device of FIGS.7(a)-7(e) after a second stage of a manufacturing process;

FIGS. 8(e)-8(h) are an end view and cross-sectional views of the deviceof FIGS. 7(a)-7(e) after a second stage of a manufacturing process.

FIG. 9 is a schematic view of a sensor assembly portion of the devicefor obtaining flow rate measurements in accordance with the invention.

FIG. 10 is a perspective schematic view of a sensor assembly portion ofthe device in accordance with the present invention.

FIGS. 11(a)-11(b) are a cross-sectional side view and an enlarged detailview, respectively, of the device of FIGS. 7(a)-7(e) in accordance withthe present invention.

FIG. 12 is a perspective view of the device of FIGS. 7(a)-7(e) inaccordance with the present invention.

FIG. 13 is a schematic representation of a fluid flow system inaccordance with the present invention.

FIGS. 14 and 14(a) are a perspective view and enlarged detail of arepresentative embodiment of a probe for use in accordance with thepresent invention.

FIGS. 15(a)-15(c) are schematic views of a representative lockingmechanism for use in accordance with the present invention.

FIG. 16 is a diagram depicting flow measurement data obtained using adevice in accordance with the present invention.

FIG. 17 is a diagram depicting air detection data obtained using adevice in accordance with the present invention.

FIG. 18 is a diagram depicting pressure sensor data for a downstreamocclusion in a fluid line using a device in accordance with the presentinvention.

FIG. 19 is a diagram depicting pressure sensor data for an upstreamocclusion in a fluid line using a device in accordance with the presentinvention.

FIG. 20 is a diagram depicting pressure sensor data for a bubbletraversing a device in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. The method and corresponding steps of theinvention will be described in conjunction with the detailed descriptionof the apparatus. The methods and apparatus presented herein are usedfor obtaining flow characteristics of a fluid flow system, such as flowrate measurements or the like. The present invention is particularlysuited for the controlled administration of beneficial agents to apatient, particularly in cases where a steady amount of beneficial agentis to be metered out over extended periods of time (e.g., days). Inaccordance with the invention, it is possible and desired to provide adevice for obtaining such measurements that is inexpensive tomanufacture and easy to use. The invention has a particular advantagefor use in intravenous (IV) applications or similar feed sets, whereinthe flow system including the reservoir and feed tube are intended to bedisposable after use.

For purpose of explanation and illustration, and not limitation, anexemplary embodiment of the device for obtaining flow characteristics inaccordance with the invention is shown in FIGS. 1(a)-1(c) and isdesignated generally by reference character 100. This exemplaryembodiment is also depicted in FIGS. 2-6. Additional embodiments areshown in FIGS. 7-8 and 11-12 for purpose of illustration and notlimitation.

For example, and for purpose of introduction only, FIGS. 1-6 show a flowsensor device 100 for obtaining flow characteristics in accordance withthe invention. FIGS. 9-10 show a sensor assembly 40 including a flowrestricting element 50, an upstream fluid pressure sensor 52 and adownstream fluid pressure sensor 56. FIGS. 1-3 show one embodiment of ahousing 10 for the sensor assembly 40 of the device. Alternativeembodiments or variations of the device, such as shown in FIGS. 7-8,also are suitable for the present invention as will be recognized fromthe description below.

The flow sensor device in accordance with the invention includes asensor assembly. The sensor assembly generally includes a first fluidflow passage having an upstream pressure sensor and a downstreampressure sensor separated by a flow restricting element.

For purposes of illustration and not limitation, the sensor assembly 40is schematically depicted in FIGS. 9-10. FIG. 9 shows a side viewrepresentation of the sensor assembly 40, while FIG. 10 shows anisometric view of the sensor assembly 40. The sensor assembly 40includes a body 42 defining a first fluid flow passage 44 having aninlet 46, an outlet 48 and a flow restricting element 50 located alongthe first flow passage 44 between the inlet 46 and the outlet 48. Asshown in FIG. 10, a registration extension 43 provides registrationbetween housing 10 and sensor assembly 40 as will be discussed. (Seealso FIGS. 5(a)-5(b), 7(d)-7(e), 8(d)). As embodied herein, the sensorassembly 40 also includes an upstream fluid pressure sensor 52 to sensean upstream fluid pressure at an upstream location 54 in the first fluidflow passage 44 between the inlet 46 and the flow restricting element50. The sensor assembly 40 also includes a downstream fluid pressuresensor 56 to sense a downstream fluid pressure at a downstream location58 in the first fluid flow passage 44 between the flow restrictingelement 50 and the outlet 48. At least one upstream signal contact 60 isconnected to the upstream fluid pressure sensor 52, and at least onedownstream signal contact 62 is connected to the downstream fluidpressure sensor 56. Preferably, the signal contacts 60, 62 are locatedon the registration extension 43 for accessibility, as will bediscussed.

In accordance with one aspect of the invention, the sensor assembly canbe constructed as an independent component, such that the body isconstructed of one or more walls as depicted in FIG. 10. As embodiedherein, the upstream pressure sensor 52 and downstream pressure sensor56 are preferably formed on a first inner surface 64 of a first wall 66of the body 42. The first inner surface 64 is substantially planar. Thedevice further includes a second inner surface 68 of a second wall 70that, as embodied herein, is also substantially planar. As embodiedherein, third wall 72 and fourth wall 74 are also provided to spaceapart first wall 66 and second wall 70. Collectively, the first wall 66,second wall 70, third wall 72 and fourth wall 74 of the sensor assembly40 cooperate to define the first fluid flow passage 44 therebetween.First wall 66 and second wall 70 are preferably formed of glass orsimilar suitable substrate. Third wall 72 and fourth wall 74 arepreferably made from silicon or the like, and can be formed on firstwall 66 and/or second wall 70 using photolithographic deposition and/orchemical etching or ion bombardment techniques as are well-known tothose of skill in the art. Upstream pressure sensor 52 and downstreampressure sensor 56 of the preferred embodiment are capacitance-typepressure sensors disclosed, for example, in U.S. Pat. No. 6,445,053titled “Micro-Machined Absolute Pressure Sensor,” the disclosure ofwhich is expressly incorporated by reference herein. Pressure sensors ofthis type are employed in a flow measurement device disclosed in U.S.Pat. No. 6,349,740, titled “Monolithic High-Performance Miniature FlowControl Unit,” the disclosure of which is also expressly incorporated byreference herein.

In accordance with an alternative embodiment of the invention (notshown), the upstream pressure sensor 52 and downstream pressure sensor56 need not be located in the first flow passage 44. For example,pressure sensors 52, 56 can be located externally to the first fluidflow passage 44 but in fluid communication with upstream location 54 anddownstream location 58 by way of pressure taps and/or fluid lines (notshown) or the like. In further accordance with this alternativeembodiment of the invention it is possible to form the body as part ofhousing to define the first flow passage 44 with flow restrictingelement 50. In this manner, the body of the sensor assembly can beformed, as will be discussed, during the insert molding process ratherthan providing a separate component.

A variety of alternative configurations and structures can be used forupstream pressure sensor 52 and downstream pressure sensor 56. Whilecapacitance-type pressure sensors are depicted herein, it is alsopossible to use other forms of differential pressure measurement. Thisis particularly applicable if pressure sensors 52, 56 are not internalto fluid flow passage 44. In accordance with this alternative aspect ofthe invention, measuring the pressure difference between upstreamlocation 54 and downstream location 58 can be achieved by any one of anumber of ways.

For example, if pressure taps (not shown) are provided at upstreamlocation 54 and downstream location 58 connected to pressuretransmission lines (not shown), each pressure transmission line can beconnected to opposite ends of a differential pressure measurementdevice. Such devices can include, for example, a liquid-filledmanometer. Alternatively, a diaphragm having one or more electricallyconductive elements disposed therein can be used to sense a differentialpressure. In accordance with this aspect of the invention, each of theupstream pressure sensor 52 and downstream pressure sensor 56 can berecognized as each of two inputs to the differential pressuremeasurement device.

As previously noted, and in accordance with the present invention, aflow restricting element is located along the first fluid flow passagebetween the inlet and outlet. With reference to FIGS. 9-10, flowrestricting element 50 is formed on the first inner surface 64 and/or onthe second inner surface 68. The flow restricting element 50 issufficiently sized and shaped to provide a proportionately largepressure drop in a flow passing through the first fluid flow passage 44over a relatively short distance, as compared to a flow passage nothaving such a flow restricting element. In this preferred embodiment,flow restricting-element 50 is preferably formed by depositing asemiconductive material such as silicon on the first inner surface 64and/or the second inner surface 68. Flow restricting element 50 can beformed integrally with first wall 66 and/or second wall 70, orseparately as an insert. Similarly, flow restricting element 50 can beprovided with a variety of alternative configurations, such as anorifice deferred through a wall or the like.

A variety of structures can be used for the structure of sensor assembly40. For example, pressure sensors 52, 56 can be provided on a glasssubstrate, which in turn is positioned in a first fluid flow passagethat is molded in the housing as a whole. Alternatively, and aspreviously discussed, a first fluid flow passage 44 could be providedthat is molded into housing 10 having pressure taps and lines in fluidcommunication with the pressure sensors 52, 56. In accordance with thisalternative aspect of the invention, first fluid flow passage 44 couldbe provided in cylindrical form. Flow restricting element 50 couldlikewise be provided in the form of an orifice that is placed in thefirst fluid flow passage 44 or integrally formed therewith. The sensorassembly 40 can be monolithic, having the flow-restricting element 50and the pressure sensors 52, 56 in an integrated structure. A monolithicsensor assembly may reduce the assembly cost and the size of the sensorassembly.

In accordance with the present invention, the flow sensor device furtherincludes a housing for the sensor assembly. The housing is configured tocontain and protect the sensor assembly, as well as ensure properinstallation within a fluid flow system.

For example and not for purposes of limitation, FIGS. 1(a)-1(c) depict ahousing 10 as embodied herein. The housing 10 has a central portion 12within which a sensor assembly 40 is contained. Housing 10 defines anupstream port 14 at an upstream end 16 of housing 10 and a downstreamport 18 at a downstream end 20 of housing 10.

Furthermore, and as embodied herein, the housing 10 also includes anupstream portion 26 and a downstream portion 28. As depicted herein, andin accordance with the invention, upstream portion 26 defines upstreamport 14 and downstream portion 28 defines downstream port 18. Althoughany of a variety of suitable configurations can be used, the portsembodied herein each includes a cylindrical bore that tapers to a narrowrectangular cross section proximate to central portion 12 so as todefine upstream flow passage 27 and downstream from passage 29,respectively.

In a preferred embodiment of the invention an upstream connector 15 islocated proximate the upstream port 14 and a downstream connector 19 islocated proximate the downstream port 18. Each connector can be providedas a flange to mate with a corresponding flange of the fluid flowsystem; however alternative connector embodiments are contemplated ifdesired. For example, Luer connectors, threaded connections or snap fitconnectors also can be used, among others. The geometry of the housing10 and connectors 15, 19 is configured to provide a seal that isadequate to prevent leakage of liquid or gaseous fluids.

Further in accordance with the present invention, the housing isprovided with at least one registration surface configured to ensureproper registration of the flow sensor device with the fluid flowsystem. Particularly, it is beneficial to ensure the inlet for a sensorassembly is registered with the upstream side (i.e. fluid source) of thefluid flow system, while the outlet of the sensor assembly is registeredwith the downstream side of the fluid flow system.

For purpose of illustration and not limitation, as embodied herein inFIG. 1(a), each of upstream engaging portion 22 and downstream engagingportion 24 is provided with one or more registration surfaces 30.Registration surfaces 30 are configured to provide alignment betweenhousing 10 and a fluid flow system as depicted in FIG. 13. When thedevice 100 is used, registration surfaces 30 ensure that the upstreamport 14 is properly aligned with a fluid source. As depicted in FIG.1(a), each registration surface 30 can be provided as a planar surfacespecifically angled to mate with a corresponding planar surface, or anyof a number of alternative configurations, such as a protrusion, a keyor a detent as shown in FIG. 7(b), provided on the fluid flow system. Aregistration surface 30 can be provided anywhere on the surface of thehousing 10. For example, a single registration surface can be providedif asymmetrical in shape or location. If registration surfaces 30 areprovided in both an upstream location and a downstream location ofhousing 10, the shape of each registration surface 30 will be differentto prevent installing device 100 backwards into a flow system.

In accordance with one aspect of the invention, and as depicted in FIGS.1(b), 1(a) and 2(b), the central portion 12 of the housing 10 defines acavity 32 of predetermined shape. As embodied herein, for purpose ofillustration and not limitation, cavity 32 is rectangular in shape. Thesensor assembly 40, which will be described in detail below, has a shapeand size corresponding with that of cavity 32. In this manner, thehousing can be fabricated separate from the sensor assembly if desired,and then later installed. Furthermore, the cavity and sensor assemblycan be provided with corresponding asymmetric shape to ensure a singleorientation between the two components. The sensor assembly can be heldwithin the cavity by a variety of mechanisms, including snap-fitconfigurations or similar mechanical connection. As a preferredembodiment, an adhesive, a bond or a weld material can be used. Cavity32 preferably has at least one surface 34 that is provided with one ormore recesses 36. As depicted in FIG. 6, recess 36 is sized to receive apredetermined amount of such material, such as adhesive, to hold sensorassembly 40 within cavity 32.

For purposes of illustration and not limitation, as embodied herein inFIGS. 1(a) and 4(a)-4(d), cavity 32 is further configured to receive acap 38. Cap 38 also has a shape and size corresponding to that of cavity32. Cap 38 has a superior face 38 a, an inferior face 38 b, end walls 38c and side wall portions 38 d. Cap 38 is placed into cavity 32 after thesensor assembly 40 has been inserted, such that inferior face 38 b isadjacent sensor assembly 40. Alternatively, sensor assembly 40 can firstbe affixed to inferior surface 38 b of cap 38, and then installed intocavity 32. As seen in FIG. 1(a), when fully inserted into cavity 32, cap38 has an external profile similar to that of the housing 10.

In accordance with yet another aspect of the invention, the cavity andcap can be used in combination to define the first fluid flow passage ofthe sensor assembly. For example, the upstream and downstream fluidpressure sensors 52, 56 can be mounted on a suitable substrate, such asglass, which is positioned within the cavity. With the sidewalls of thecavity defining side walls of the first fluid flow passage 44, the capis positioned in the cavity and appropriately spaced from the sensors52, 56 to complete the fluid flow passage 44. If desired the flowrestricting element can be formed on the inferior surface 38 b of thecap, or provided as a separate element.

Housing 10 preferably is made of a plastic that is injection-moldedinside a molding cavity. Particularly, the housing can be made fromacrylic, Cryolite, or a composite fiber-reinforced material, althoughany other suitable material-including metals and ceramics, can be used.If plastic is used the housing 10 is preferably formed by liquidinjection insert molding. As is known in the art, insert molding for ahollow member generally involves using removable inserts within amolding cavity to prevent the flow of liquid plastic materials intopreselected volumes within the cavity in order to define voids in thefinished article. It is recognized, however, that alternativetechniques, such as milling or machining, can be used if desired.

An advantage of the housing 10 depicted, for example, in FIG. 1(a), isthat it can be manufactured generally by a single injection of plasticmaterial into a mold. In this manner, inserts or “slides” are providedin a molding cavity to define voids to be created for cavity 32,upstream flow passage 27 and downstream flow passage 29. Next, liquidplastic material is injected into the mold, filling all open spaces.After hardening, the slides are removed and housing 10 is removed fromthe mold. The end result is a housing 10 as depicted in FIG. 3. Thecavity 32 provided in this first representative embodiment of housing 10enables the sensor assembly 40 to be installed in housing 10, asdescribed above, followed by installation of cap 38 as depicted in FIG.1(a). Cap 38 is also preferably made from an injection-molded plasticmaterial such as Cryolite or acrylic, but can also be made from otherplastic materials, composite materials or metal, if desired.

In further accordance with the invention, the housing defines a probeaccess port configured to provide access of a probe to at least one ofthe upstream signal contact and downstream signal contact.

For purposes of illustration and not limitation and with specificreference to FIGS. 1(a), 3, 5 and 14, as embodied herein, probe accessport 39 is defined by a gap between cap 38 and sensor assembly 40. Probeaccess port 39 provides access of a probe 90 to at least one of upstreamsignal contact 60 or downstream signal contact 62, and preferably toboth, on surface 43. The physical geometry of probe access port 39provides alignment between signal contacts 60, 62 and an external probe90 as discussed below. Generally, however, probe access port 39 can beof any desired configuration that provides suitable registration betweensignal contacts 60, 62 and probe 90.

Particularly, and in accordance with another aspect of the inventions,the probe access port 39 has a shape and size corresponding to apredetermined shape and size of the probe to ensure proper alignment ofthe probe with the corresponding contacts 60, 62. A preferred embodimentincludes using a wedge shape for the predetermined shape of theconnector body of the probe 90 and corresponding port 39. The contacts60, 62 are located on the proximate port 39, the apex of the wedgeshape, and the leads 92 of probe 90 are located on the apex of theconnector body. In this manner, the angled surfaces of the wedge shapesinteract to align more accurately the leads 92 with the contacts 60, 62as shown in FIG. 14. Thus, contact is made between probe 90 and onelongitudinal surface 39 a and one radial surface 39 b that define probeaccess port 39 within housing 10 that provides for adequate force toassure contact between leads 92 on probe 90 and contacts 60, 62 onsensor assembly 40. These electrical contacts are preferably in closeproximity to registration surfaces 30 on the outside of housing 10 thatare engaged with external clamp assembly 120 that is also preferablyreferenced to the probe 90 (See FIGS. 15(a)-15(c)).

A variety of different configurations can be used for probe access port39. Port 39 can alternatively be slot-shaped or can take other forms, solong as the geometry of housing 10 provides for registration andalignment between signal contacts 60, 62 and leads 92 on probe 90.

In accordance with another aspect of the invention, the housing canfurther define a second fluid flow passage between the upstream port anddownstream port of the device.

For purposes of illustration and not limitation, FIGS. 7(a)-7(e) and8(a)-8(h) show a second exemplary embodiment of a flow measurementdevice in accordance with the invention. As embodied herein, housing 10includes a second fluid flow passage 80. The second fluid flow passage80 provides a flow path that is arranged for fluid communication inparallel to the first fluid flow passage 44 between the upstream port 14and the downstream port 18. In this manner, second fluid flow passagecan act as a bypass line in combination with first fluid flow passage.The embodiment is particularly beneficial when increased fluid flow isrequired past the flow restricting element of the sensor assembly.

Preferably, a valve is provided in operative communication with thesecond fluid flow passage for selective fluid flow therethrough. Any ofa variety of suitable valve configurations can be provided. In apreferred embodiment, however, and as shown in FIG. 8, the valve isformed of a compressible wall member 82 of the second fluid flow passage80. The compressible wall member 82 is preferably formed from anelastomeric material such as silicone.

The second fluid flow passage 80 further has a first transversedimension 84 and a second transverse dimension 86 perpendicular to thefirst transverse dimension. (See FIG. 8(e)). As embodied herein, thefirst transverse dimension 84 is smaller than the second transversedimension 86, such that the cross section of the second fluid flowpassage has an ellipsoidal shape with a small radius at each apex 87(See FIGS. 8(d), 8(e), 8(h)). In this manner, the compressible wallmember 82 is more readily compressed upon the application of a forcealigned with the first transverse dimension, than if the second fluidflow passage had a circular cross section. Moreover, the small radius ofeach apex 87 ensures that the second fluid flow passage can close with aminimal applied force.

It is noted that a modified manufacturing process is used when forming adevice in accordance with the second representative embodiment of theinvention of FIGS. 7(a)-7(e) and FIGS. 8(a)-8(h). When forming a housing10 with a compressible wall member 82, the housing 10 is formed indistinct manufacturing steps.

To make the embodiment of housing 10 depicted in FIGS. 7, 8, 11 and 12,the sensor assembly 40 preferably is first placed between slides withina mold, wherein the slides define voids to be created for second fluidflow passage 80 and surrounding elastic wall member 82, upstream flowpassage 27, downstream flow passage 29 and the probe access port 39.Next, the desired liquid plastic material is injected into the mold,filling all open spaces as described above. After hardening, the housinghas a form as depicted in FIGS. 7(a)-7(e). The slide(s) defining thevoids to be created for second fluid flow passage 80 and surroundingelastic wall member 82 are replaced with smaller slide(s) correspondingto the size and shape of second fluid flow passage 80. As embodiedherein, an elongate slide with an elliptical cross section with a smallradius at the apex 87 can be used. The slides defining upstream flowpassage 27 and downstream flow passage 29 are also retracted slightly,to create disc-shaped voids in the upstream flow passage and downstreamflow passage 29 near central portion 12. Next, a suitable liquidelastomeric resin is injected into the voids to form the elastic wallmember 82 of second fluid flow passage 80, and disc-shaped seals 85 inthe upstream flow passage and downstream flow passage 29 near centralportion 12. After the elastomeric material cures, housing 10 is removedfrom the mold. The structure that results from this manufacturingprocess is depicted in FIGS. 8(a)-8(h). Seals 85 can assist in providinga liquid and gaseous seal between device 100 and a fluid flow line 102.

As further depicted in FIGS. 11(a)-11(b), the sensor assembly can besecured with the housing so as to protrude into flow passages 27, 29.This facilitates the manufacturing process, since slides will generallybe used to hold sensor assembly 40 in position during the manufacturingprocess. As an alternative however, the sensor assembly can bepositioned subsequently within a cavity formed in the housing in amanner similar to that described with regard to FIGS. 1-3 above ifdesired.

In accordance with another aspect of the invention, a fluid sensorsystem is provided. The system includes a device for obtaining flowcharacteristics as described above as well as a probe to receive signalsrepresentative of a fluid flow characteristic and a processor to processsignals from the probe.

For purposes of illustration and not limitation, as embodied herein andwith reference to FIG. 13, a system is depicted schematically includingflow measurement device 100 in accordance with the invention asdescribed above in combination with a probe 90 and a processor 110.

As previously discussed, one aspect of the invention includes providingthe probe with a connector body having a predetermined shape wherein theprobe access port of the housing has a corresponding shape to ensureproper alignment of the probe with at least one of the upstream signalcontact and downstream signal contact.

For example, and as embodied herein, probe 90 has a wedge-shapedconnector body that corresponds to the shape of probe access port 39 asdepicted in FIG. 14. Advantageously, the geometry of probe access port39 (perimeter of opening indicated by dashed lines) eliminates any needfor a lead frame to provide registration between probe 90 and sensorassembly 40.

Particularly probe 90 includes a connector body 95 having a plurality ofleads 92 that are connected to a processor as discussed below. Withreference to FIG. 14, the corresponding shapes of probe access port 39and connector body ensure proper registration between contacts 60, 62 onthe sensor assembly 40 with leads 92 on the probe 90. Preferably, thegeometric tolerance between probe 90 and probe access port 39 issufficiently small to permit probe 90 to be press-fitted or snap fittedinto probe access port 39. Leads 92 can be further configured to wipeacross contacts 60, 62 while being inserted as depicted in FIG. 14.Providing a wiping action ensures a stable fit and good electricalcontact between leads 92 and contacts 60, 62. Thus, as discussed above,contact is made between probe 90, surface 39 a and surface 39 b toprovide for adequate force to assure contact between leads 92 on probe90 and contacts 60, 62 on sensor assembly 40. The purpose of this is toassure precision in locating the multiple contacts 92 on probe 90 withthe contacts 60, 62 on sensor assembly 40. The fit between leads 92 andcontacts 60, 62 must be secure to ensure a connection that does notgenerate excessive noise that would reduce the sensitivity of thesystem. Contacts 60, 62 are preferably made of gold, although othersuitable electrically conductive materials can be used.

Probe 90 is preferably a flexible printed circuit element. Morepreferably, the probe includes a plurality of signal leads 92 locatedbetween two or more conductive shield layers 96 that are insulated fromthe signal leads 92 to minimize noise. The signal leads 92 defined bythe flexible printed circuit element will further define or separatelyinclude a spring element for enhanced contact. To prevent damage to thespring based leads 92, however, the connector body 95 is configured toprevent over bending of the leads beyond an established limit. This isaccomplished by containing the leads within a gap 98 of sufficientclearance defined in the connector body 95, as shown in detail of FIG.14(a). The connector body of the probe can be over molded of anysuitable material, such as plastic or elastomeric, or formed byalternative known techniques, to protect the signal leads.

A variety of alternative configurations and structures can be used forprobe 90. For example, although probe 90 is depicted herein as a singleflexible printed circuit element, a plug (not shown) with a plurality ofconductive prongs can be used, wherein the probe access port 39 isdefined by a plurality of passages (not shown) through housing 10configured to provide registration between electrical contacts 60 onsensor assembly 40 and the plurality of conductive prongs on probe 90.

In accordance with a further aspect of the invention, the system furtherincludes a fluid flow system comprising a fluid flow line incommunication with a fluid source. As embodied herein and with specificreference to FIG. 13 for purpose of illustration, a fluid flow line 102is provided in communication with a fluid source 104. The fluid source104 can be a pump 106 connected to a reservoir 108. In accordance withthis aspect of the invention, pump 106 is used to selectively pump fluidthrough the first flow passage using positive displacement of the like.

A variety of alternative configurations can be used for fluid source104. For example, fluid source 104 can include a conventionalintravenous feed reservoir, such as a bag or bottle, connected to fluidflow line 102 for gravity feed. Preferably, a control valve (not shown)is provided in series with fluid flow line 102 for control of the flowby a processor (discussed below) in response to signals from device 100to increase or decrease the rate of flow. The pump and/or control valvecan be adjusted manually or automatically.

As previously noted, the system includes a processor to process signalsreceived by the probe. The processor can be provided in a variety offorms, such as a software program for operation on a conventionalworkstation, or as hardware embedded into a chip or on a hardwireddevice as is known in the art.

In accordance with a further aspect of the invention, the processorcontrols the pump in response to signals obtained by the probe from thesensors.

For purposes of illustration and not limitation, with specific referenceto FIG. 13, a system is provided including a processor 110. Processor110 can be a control circuit that is programmed to vary the flow outputof pump 106 in response from signals obtained from upstream fluidpressure sensor 52 and downstream fluid pressure sensor 56 to provide adesired rate of fluid flow. Alternatively, processor 110 can be providedin the form of a computer workstation (not shown). Examples of suitableprocessors are a wide variety of embedded processors available from manysemiconductor manufacturers such as Intel Corporation, Advanced MicroDevices, Inc. (“AMD”) and Integrated Device Technology, Inc. (“IDT”).

In accordance with yet a further aspect of the invention, the system canfurther include a locking mechanism to mate the housing with the fluidflow line. Generally, the locking mechanism at least has an unlockedcondition for receipt of the housing, and a first locked condition toalign the housing with the fluid flow line. In a preferred embodiment,the locking mechanism further includes a second locked condition toposition the probe in the housing.

The locking mechanism can be provided in any of a variety of forms orconfigurations. For example, one or more lever members can be provided,each with a first condition to allow receipt of the housing 10 intocommunication with the fluid flow line 102, and a second condition toalign and secure the housing in position. The probe 90 can be mounted onone such lever member so as to be inserted into the probe access port 39and in communication with the contacts when the lever member is moved toits second condition.

For purposes of illustration and not limitation, as further embodiedherein and depicted schematically in FIGS. 15(a)-15(c), a lockingmechanism 120 is provided to connect the housing with a fluid flow line102. Locking mechanism 120 is defined by a locking body 122, and a levermember defined in this embodiment as cover 130. Cover 130 has two hinges132 and 134. Hinge 132 connects locking body 122 with a first coverportion 136 of cover 130. Hinge 134 connects first cover portion 136 ofcover 130 to second cover portion 138 of cover 130.

In an unlocked condition, as depicted in FIG. 15(a), the lockingmechanism can receive flow measurement device 100. Flow measurementdevice 100 is placed in the locking mechanism 120 when the lockingmechanism 120 is in an unlocked condition wherein cover 130 is fullyopen, such that registration surface 30, defined as one or more detents,mate with receiving surface 124, defined by corresponding protrusions.

Locking mechanism can be changed from the unlocked condition to a firstlocked condition. As embodied herein and as depicted in FIGS.15(a)-15(b), locking mechanism 120 is changed to the first lockedcondition by rotating (in direction of arrow “A”) first cover portion136 of cover 130 about hinge 132 so that tabs 135 on cover 130 mate withtabs 125 on locking body 122. Preferably, a snap fit is provided,although alternative closure mechanisms can be used if desired. Thus, inthe first locked condition, locking mechanism 120 holds housing 10 offlow device 100 in place in-fluid flow line 102, such that registrationsurface 30 on the housing 10 is maintained in alignment with receivingsurface 124. Locking mechanism 120 thus ensures alignment between fluidflow line 102 and flow sensor device 100.

The locking device can further include a second locked condition. Forpurposes of illustration and not limitation, as embodied herein in FIGS.15(b)-15(c), locking device 120 is changed from a first locked conditionto a second locked condition by rotating second cover portion 138 (indirection of arrow “B”) about hinge 134 until tabs 137 on second coverportion 138 engage with tabs 127 on locking body 122. If desired, thesecond cover portion 138 can be connected by a hinge to the locking bodyfor independent operation, for example along the longitudinal edge 134′opposite hinge 132, such that first cover portion 136 and second coverportion 138 can be operated independently.

In a preferred embodiment, and as best seen from FIG. 15 c, the probe ismounted to or otherwise attached to second cover portion 138, such thatmovement of second cover portion 138 into the second locked condition oflocking mechanism advances probe 90 into probe access port 39 in housing10. As embodied herein, second cover portion 138 defines an opening 138a that fits around and provides registration with probe 90. Second coverportion 138 can then retain probe 90 in place to ensure secure contactfor making fluid flow measurements or the like.

In accordance with a further aspect of the invention, as previouslydescribed with regard to the embodiment of FIGS. 7-8, a valve can beprovided for selective flow through a second fluid flow passage. Thevalve has a first condition to allow flow through the second flowpassage and a second condition to prevent flow through the second flowpassage. The system of the preferred embodiment further includes anactuator to change the valve from the first condition to the secondcondition when the locking mechanism is moved from the first lockedcondition to the second locked condition.

For purposes of illustration and not limitation, as embodied herein, thesecond exemplary embodiment of FIGS. 7-8 in accordance with theinvention is shown in FIG. 15 having a second fluid flow passage 80. Thevalve includes at least a portion of the second fluid flow passage,wherein the valve is defined by a compressible wall member. As embodiedherein, the second fluid flow passage 80 is, by default, in a firstcondition, or an open state such that fluid can be passed therethrough.After having been placed in the locking mechanism 120, it is possible toprovide one of the cover portions with an actuator embodied asprotrusion 139, wherein protrusion 139 presses against compressible wallmember 82 of second fluid flow passage 80, so as to move the value tothe second condition. As discussed above, the cross-section of secondfluid flow passage 80 is preferably elliptical with a small radius atthe apex. The dimension of flow passage 80 parallel to the line of forceexerted by protrusion 139 is less than the dimension of flow passage 80that is perpendicular to the line of force of protrusion 139. In thismanner, relatively less force is required to compress the compressiblewall member and thus close the valve. As embodied herein, protrusion 139is a pin although alternative actuators can be used depending on thevalve.

In accordance with a further aspect of the invention, if desired, secondfluid flow passage 80 can be opened by opening the valve to increaseflow through the flow measuring device 100. This could be accomplishedby opening the appropriate cover portion or by configuring the actuator,e.g. protrusion 139, for independent movement such that it can be movedto a position where it does actuate the valve.

A variety of structures can be used for the protrusion 139. For example,a spring-loaded pinch valve (not shown) can be used. Alternatively, thesecond fluid flow passage 80 can be made of an elastic material that isbiased to remain closed, whereby the resistive force of the passage canbe overcome by an increase in fluid pressure or by application of alateral force to open the elliptical passage. Additionally oralternatively, a frangible membrane (not shown) can be provided, toinitially block the second fluid flow passage 80, which in turn could beruptured by an actuator or by a pressure surge should it becomenecessary to deliver a significant amount of beneficial agent to apatient through device 100 in a relatively short amount of time.

In further accordance with the invention a method is provided forobtaining flow characteristics of a fluid flow system. The methodincludes providing a device described above; directing a fluid flowthrough the first fluid flow passage; obtaining a signal correspondingto the fluid pressure in the first fluid flow passage at the locationsof the upstream fluid pressure sensor and the downstream fluid pressuresensor; and determining a flow characteristic based upon the signal. Themethod has been described in detail in conjunction with the device andsystem of the invention.

As embodied herein and with reference to FIGS. 9 and 10, a fluid can beflowed through first fluid flow passage 44 by applying a differentialfluid pressure across the inlet 46 and outlet 48 of the sensor assembly40. When fluid flows through the first flow passage 44, a differentpressure reading is detected at an upstream location 54 than at adownstream location 58. This difference in fluid pressure reflects thatthe fluid flow has lost energy between location 54 and location 58 dueto frictional interactions with the surfaces of first flow passage 44,particularly flow restricting element 50. These losses can beempirically correlated to a volume flowrate of a given fluid through thefirst flow passage 44 at a selected temperature. A variety of sensorsfor obtaining such information are known; in a preferred embodiment,however, a capacitive pressure sensor is used.

EXAMPLE I Flow Measurement

As embodied herein, each capacitive pressure sensor 52, 56 is used tomeasure pressure by detecting the change in capacitance of the pressuresensor. This measurement is accomplished by applying a voltage acrosseach pressure sensor 52, 56. A voltage signal is then generated that isindicative of the capacitance of the pressure sensor, and thereforeindicative of the pressure in the flow at either upstream location 54 ordownstream location 58 at a particular point in time. Signals obtainedfrom each pressure sensor 52, 56 are routed to processor 110. FIG. 16depicts signal levels over time from each pressure sensor. The upstreampressure sensor output is indicated by 151 and the downstream pressuresensor output is indicated by 152. As shown, the signal levels indicatedby 151 and 152 are separated by a voltage level difference. The signallevel difference, indicated by ΔV, is indicative of the pressure drop,and thus flowrate, between upstream location 54 and downstream location58.

Since the relative voltage obtained from each of the pressure sensors52, 56 is indicative of a differential pressure, it is possible toempirically establish the flowrate of fluid by a given difference involtage output between pressure sensor 52 and pressure sensor 56.Moreover, if the Reynold's number of the flow is known for the empiricalcase, or if the viscosity, density and/or temperature are known of thefluid, then additional flow characteristics can be determined orcalculated using known techniques. Thus, based on empiricalexperimentation and information, desired flow characteristics of fluidthrough the first fluid flow passage can be determined based on receivedvoltage signals from the pressure sensors, as well as from the physicalproperties of the fluid that are known or can be closely estimated.

In accordance with a further aspect of the invention, the determiningstep can include determining the pressure difference between theupstream and downstream fluid pressure sensors. The determining step canfurther include calculating or otherwise determining a flow rate offluid through the first fluid flow passage based on the pressuredifference rather than the signal measurements.

The method of the invention further includes the step of determining theactual pressure difference between the upstream pressure sensor 52 andthe downstream pressure sensor 56 instead of empirically correlating theoutput signal levels directly with flowrate. Flow passes through firstflow passage 44 of sensor assembly 40 by passing inlet 46, upstreamlocation 54, flow restricting element 50, downstream location 58 andoutlet 48 as seen in FIGS. 9-10. Although it is not actually necessaryto convert the signal output to a pressure reading before determining aflowrate through first fluid flow passage 44 of sensor assembly 40,circumstances can arise that make obtaining an actual pressuremeasurement desirable. For example, knowing the total pressure at thelocation 54 of the upstream pressure sensor 52, or at the location 58 ofthe downstream pressure sensor 56, or the differential pressure acrossflow restricting device 50 can be useful if the system is being operatedat a condition that requires monitoring. Rates of fluid flow throughfirst flow passage 44 can then be calculated based on the calculatedpressure difference measured by upstream pressure sensor 52 anddownstream pressure sensor 56.

EXAMPLE II Air Detection

In accordance with another aspect of the invention, the determining stepincludes detecting the presence of air in the first fluid flow passage.The step of detecting air in the first fluid flow passage can includeidentifying convergence of the signal received from the upstream fluidpressure sensor and the signal received from the downstream fluidpressure sensor.

As embodied herein, the step of determining the presence of air in thefirst fluid flow passage 44 includes determining when the pressuredifference measured by pressure sensors 52, 56 approaches zero.

FIGS. 16-17 depict a signal output for each pressure sensor 52, 56 whena 50 microliter bolus of air is being detected in first flow passage 44.The first signal trace 151 (in FIG. 16) and 156 (in FIG. 17) which is anexpanded time scale of the same data is identified as above 0 units, andthe second signal trace 158 is below zero units, wherein each unit canbe a measure of voltage or of relative pressure. During a normal liquidflow, the signal levels are several units apart as described above.However, when air is injected into the line, the voltage signalsconverge toward each other reflecting a drop in pressure differentialand the presence of air in the flow line. The signal paths convergebecause the presence of a gas entrained in the liquid has a largedifference in fluidic resistivity that causes the pressure to equalizewhile the air traverses the restriction rapidly and initiates a pressureshock wave in the downstream location 58 of the first flow passage 44.

For example, as a volume of air (i.e., a bubble) passes through thesensor assembly, the bubble envelopes both the upstream and downstreampressure sensors causing the pressure difference to approach zero. Thevolume of the sensor assembly can be small, such that very small bubbles(about 1 microliter) can be detected. As air passes over the sensors,the rapid change in fluidic resistance also generates substantialtransient spikes. By monitoring these transients, a bubble can bedistinguished from an upstream occlusion. In a preferred embodiment, theupstream pressure sensor 52 and downstream pressure sensor 56 arecapacitance-type pressure sensors disclosed, for example, in U.S. Pat.No. 6,445,053 titled “Micro-Machined Absolute Pressure Sensor” that canbe positioned less than 1 mm apart. This embodiment of a small, dualsensor assembly allows the detection and measurement of bubbles as smallas 1 μl. Previous fluid flow measurement systems detected bubbles on theorder of 50 μl, but could not accurately measure the size of thebubbles. In a preferred embodiment of the present invention, bubbles maybe detected that are 1 μl and larger. Generally, the system would detectbubbles in the range of 1-50 μl. Larger bubbles would be detectable, butare limited merely by the time measurement of the system.

The extent of convergence of the signals is generally indicative of theamount of air detected in the flow passage. That is, the pressuredifferential will drop to zero when the passage is essentially filledwith air. In a sensor assembly having a small volume, bubbles passthrough the assembly rapidly. As a bubble passes out of the sensorassembly, the upstream pressure, P1, is restored to its initial flowvalue. By measuring the time the bubble traverses the device, Δt, asshown in FIG. 21, the size of the bubble can be determined. The bubblevolume can be calculated according to the following formula:Bubble volume=velocity of the bubble×Δt×cross sectional area of the flowpathFor example, it is possible to quantify the actual volume of air whenthe system includes an upstream pump that is controlling the actual flowof fluid.

EXAMPLE III Pulsed Flow

In accordance with yet another aspect of the invention, a method isprovided further including the steps of intermittently pulsing the fluidthrough the first fluid flow passage and detecting the fluid pressure inthe first fluid flow passage using the upstream fluid pressure sensorand the downstream fluid pressure sensor to determine the amount offluid delivered each time the pump is pulsed. This method of obtainingflow characteristics is particularly useful when the flow rate throughthe fluid flow system is sufficiently low, such that background noisewill interfere with signal measurements of a continuous flow.

In accordance with this aspect of the invention, a data signal output(not shown) similar to that in FIG. 16 occurs, except that it indicatespulsed operation evidenced by each signal trace gaining amplitude,dropping to zero, and then repeating transient as expected with periodicflow. The area under the signal curve can be integrated and empiricallycorrelated to a set volume of fluid flowed through first fluid flowpassage over a selected period of time. This is advantageous if it isdesired to deliver extremely low dosages of a beneficial agent to apatient over an extended period of time, since a lower, steady flow overthat time would not create a differential pressure signal that issufficiently high to detect.

Specifically, when implementing a method in accordance with this aspectof the invention, it is useful to enable the flow sensing andintegrating functions to only receive and compile signals received frompressure sensors 52, 56 during short bursts of fluid flow and anyassociated transients. Varying the average flow over a large range ofdelivery rates by varying the time period between the short bursts offluid flow can assist in calibration of the system, and ensure accurateoperation.

In accordance with still another aspect of the invention, a method isprovided wherein the housing provided by the housing step includes asecond fluid flow passage and a valve for selection of flow through thesecond fluid flow passage. The valve has a first condition to allow flowthrough the second flow passage and a second condition to prevent flowthrough the second flow passage. The method further includes the step ofopening the valve to increase flow through the housing.

As described above, and for purpose of illustration only, housing 10 canbe provided with a second fluid flow passage 80. When flow sensor device100 is placed in locking assembly 120 in the second locked condition,protrusion 139 causes second fluid flow passage to close, as depicted inFIG. 15(c). By opening valve 140, such as by opening cover member, it ispossible to increase flow through housing 10 to prime the third flowsystem and purge all air, or in case of a patient emergency, or othercircumstance warranting a rapid increase in flowrate.

EXAMPLE IV Occlusion Detection

In accordance with yet another aspect of the invention, the sensorassembly can be used to determine if there are occlusions, includingpartial occlusions, in a fluid line, and the location of the occlusions.As shown in FIG. 18, an upstream occlusion in a fluid line can bedetected when the upstream pressure sensor detects a reduction inpressure while the downstream pressure sensor detects a relativelysteady pressure. As shown in FIG. 19, a downstream occlusion in a fluidline can be detected when the upstream pressure sensor detects arelatively steady pressure and the downstream pressure sensor detects anincrease in the downstream pressure.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the device, method andsystem of the present invention without departing from the spirit orscope of the invention. Thus, it is intended that the present inventioninclude modifications and variations that are within the scope of theappended claims and their equivalents.

1-39. (canceled)
 40. A variable rate infusion device, comprising: afluid flow line in communication with a fluid source including a flowcontrol means and a reservoir; and a variable flow restriction devicecoupled to the fluid flow line and including a first fluid passage witha flow restricting element therealong, a second fluid passage fluidly inparallel with the first fluid passage, and a valve for selectivelyadjusting the second fluid passage to vary fluid flow through thevariable flow restriction device.
 41. A variable rate infusion deviceaccording to claim 40, wherein the flow control means is a positivedisplacement pump.
 42. A variable rate infusion device according toclaim 40, wherein the flow control means is a control valve in serieswith the fluid flow line so as to control fluid fed by gravity.
 43. Avariable rate infusion device according to claim 40, further comprisinga housing containing the second fluid passage and a flow sensor assemblythat defines the first fluid passage and the flow restricting element.